We propose continuation of a highly successful Cooperative Hepatitis C Research Center involving a consortium of investigators. Project 1 is directed by Dr. S.M. Lemon (Center Director, UTMB) and focuses on the role of the NS3/4A protease in disrupting cellular signaling pathways leading to induction of type 1 interferons. The project seeks to identify novel cellular NS3/4A substrate proteins that participate in these pathways, to determine how sequence variation within NS3/4A influences its ability to disrupt signaling, and to utilize this information to develop cell lines that are more permissive for viral replication. Project 2 is directed by Dr. R.E. Lanford (SFBR) and will characterize acute and chronic infection events in the chimpanzee, including studies that integrate sophisticated immunohistochemical imaging of gene expression in situ with previous microarray analyses, and determine the rates of hepatocyte death and proliferation once the infection becomes persistent. Dr. Lanford will also characterize correlates of cross-genotype immunity and investigate the molecular basis for genotype-specific resistance to interferon-alpha. Project 3, directed by Dr. S. Ray (JHU), will characterize events during acute infection in humans that ultimately lead to T lymphocyte failure. Dr. Ray will assess molecular markers of CD8+ T cell failure in cells derived from both peripheral blood and liver and examine the role of CD4+ cells in modulating CD8+ T cell responses during acute HCV. Project 4 is directed by Dr. M. Gale (UTSW) and will investigate the role of innate intracellular defenses in regulating HCV replication, and whether viral persistence is linked to HCV disruption of the retinoic acid inducible-gene I (RIG-I) pathway signaling induction of host defenses. Core A, Administrative Core, directed by Dr. Lemon (UTMB), will provide oversight and facilitate the integration of these highly interactive and collaborative projects with the national Hepatitis C Research Center Network. Core B, Clinical Core, directed by Dr. D. L. Thomas (JHU),will prospectively follow an established cohort of HCV-naive intravenous drug users to provide unique clinical specimens to be used in each of the Center's projects.